CLASSICAL, NONCLASSICAL, AND MIXED-METAL OSMIUM(II) POLYHYDRIDES STABILIZED BY THE TETRAPHOSPHINE P(CH2CH2PPH2)3 - H D ISOTOPE-EXCHANGE REACTIONS PROMOTED BY A STRONGLY BOUND DIHYDROGEN LIGAND/

The cis-hydride-eta2-dihydrogen complex [(PP3)OsH(eta2-H-2)]BPh4 (4), PP3 = P(CH2CH2PPh2)3, has been prepared by either protonation of the d ihydride [(PP3)OsH2] or reaction of the novel dinitrogen complex [(PP3 )OsH(N2)]-BPh4 (6) with H-2. The nonclassical structure of 4 has been established in solution by variable-temperature NMR spectroscopy (H-1, P-31), T1 measurements, and J(HD) values. An X-ray diffraction analys is has shown that the solid-state geometry of the compound is not inco nsistent with the nonclassical assignment. Crystals of 4 are triclinic , space group P1BAR with a = 16.841(4) angstrom, b = 15.135(2) angstro m, c = 12.450(3) angstrom, alpha = 91.53(2)degrees, beta = 96.07(2)deg rees, gamma = 106.75(2)-degrees, V = 3016.23 angstrom3, and Z = 2. In the solid state and in solution at low temperature, [(PP3)OsH(eta2-H-2 )]+ is octahedral and the hydride and dihydrogen ligands occupy mutual ly cis positions. The H2 ligand is trans to the bridgehead phosphorus Of PP3. At ambient temperature, 4 is highly fluxional. The dynamic beh avior of the complex in solution has been studied by variable-temperat ure P-31{H-1] NMR spectroscopy using the DNMR3 program. A comparison o f the chemical properties, in particular H-2-displacement reactions, w ithin the series [(PP3)MH(eta2-H-2)]+ (M = Fe, Ru, Os) leads to the co nclusion that the osmium complex has the strongest interaction with th e H-2 ligand. Complex 4 has been found capable of reacting with (CD3)2 CO to give the perdeuterated isotopomer 4-d3 via a keto-enol tautomeri zation reaction of acetone-d6. The isolation and characterization of t he mixed osmium-gold dihydride [(PP3)OsH{eta2-HAu(PPh3)}]PF6 has provi ded experimental evidence for the correctness of the H-2 and HAu(PPh3) isolobal relationship. The dinitrogen ligand in 6 can easily be displ aced by a plethora of ligands, including weak ones such as THF and ace tone. The latter reagent reacts with 6 to form the first example of a stable and isolable cis-hydride-eta1-ketone complex, [(PP3)OsH{eta1-OC (CH3)2}]BPh4, a suggested key intermediate in metal-assisted hydrogena tion reactions of ketones.